1. Field of the Invention
This invention relates to a mobile telephone or portable telephone system (cellular system) which uses a direct sequence code division multiple access (DS-CDMA) communication method, and more particularly to a path search circuit for a base station radio apparatus.
2. Description of the Relates Art
Mobile communications systems in which a CDMA communication method is used have been developed in recent years and include systems which are based on the IS-95 standards (TIA/EIA) and have been put into practical use already and W-CDMA (Wideband Code Division Multiple Access) systems which are third generation mobile communications systems whose standardization is being proceeded in the 3 GPP (3rd Generation Partnership Project) although they have not been put into practical use as yet.
In a system based on the IS-95 standards, a spread code obtained by multiplying a PN code having a comparatively long period of 26.6 ms (80 ms/3, 32768 chips) and a Walsh code of a code length of 64 is used as a spread code for a downlink which is a link from a base station to a mobile station. As the PN code, different codes (accurately, codes shifted by predetermined number of times from the same spread code) are used by different base stations and, even in the same base station, for different sector antennae. The Walsh code of the code length of 64 is used to distinguish a plurality of channels transmitted from one sector antenna (for the CDMA, the same carrier is shared by a plurality of channels and the channels are distinguished with the spread code). A pilot channel which is not modulated with data is transmitted with a comparatively high power for each sector. For the Walsh code used in the pilot channel, the 0th code, i.e., a code of all xe2x80x9c0xe2x80x9ds, is used. In other words, a signal transmitted by the pilot channel is a predetermined code sequence having a period of 26.6 ms. Accordingly, a mobile station of a system which is based on the IS-95 standards uses the pilot channel to detect a peak of a cross-correlation between a predetermined code sequence of the pilot channel and a received signal to detect a path timing. The period of the spread code is 32,768 chips and is too long to determine cross-correlation coefficients at a time. Therefore, a sliding correlator is used to successively determine correlation coefficients while the received signal and a reference signal (the predetermined spread code of the pilot channel) are successively shifted in time.
A conventional reception timing detection method (chip synchronization) is disclosed in the following reference document 1, for example:
Reference document 1: Andrew J. Viterbi, xe2x80x9cPrinciple of Spread Spectrum Communicationxe2x80x9d, April 1995, Chapter 3, pp. 39-66, FIGS. 3.1, 3.2 and 3.6
According to the reception timing detection method disclosed in the reference document 1, acquisition of a timing of a signal spread with a spread code, which is a pseudo random code, is performed in two stages. In particular, the acquisition is divided into two stages of initial synchronization acquisition (search) and synchronization tracking (tracking). An initial synchronization acquisition (search) method is a method of serially searching for a timing while the reception timing is successively displaced by a xc2xd chip interval until the correlation power exceeds a certain threshold value as recited in Chapter 3, Paragraph 4 of the reference document 1. The synchronization tracking (tracking) is performed by a method called early-late gate or DLL (Delay Lock Loop). According to the methods, a correlation power at a timing earlier by xcex94t of a delay time than a timing at which the signal is to be received and another correlation power at another timing later by xcex94t are determined, and the timing is finely adjusted so that the difference between the correlation powers may be reduced to 0.
After a path timing is detected once, it is only required that a variation of the path timing can synchronously follow up (tracking) the variation of the propagation time between a base station and a mobile station which is caused by a movement of the mobile station and the variation of the propagation time caused by a positional relationship with a reflecting object or objects in multi-path propagation paths. Therefore, a cross-correlation coefficient (which represents a delay profile of the propagation path) may be determined within the range of several microseconds to several tens microseconds before and after the present timing. Determination of a cross-correlation coefficient (delay profile) within the range limited in this manner can be realized also by a plurality of correlators which operate simultaneously. Where it can be regarded that the path timing varies continuously in time, also a timing follow-up method in which a DLL is used as disclosed in the aforementioned reference document 1 has been realized.
Various standards for the W-CDMA are recited in the following reference document 2:
Reference document 2: 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Spreading and modulation (FDD), 3G TS 25.213 version 3.1.0, December 1999
According to the reference document 2, the W-CDMA uses a Gold code of a period of 10 ms as a spread code and a Walsh code of a period of 1 symbol (the code length varies depending upon the symbol rate). In the downlink, different Gold codes are used in different base stations and for different sectors in the same base station. In the uplink from a mobile station to a base station, Gold codes different among different mobile stations are used, and different Walsh codes are allocated to different physical channels in the same mobile stations. In both of the uplink and the downlink, a pilot symbol modulated with a predetermined code sequence is multiplexed (code multiplexed and time multiplexed).
Different from the downlink in a system based on the IS-95 standards, a pilot symbol of the W-CDMA is not spread with the same spread code (including shifted spread codes) among all base stations or all mobile stations. However, if the spread code is known, then a code sequence of the pilot symbol can be considered as a completely known code sequence. Accordingly, in the W-CDMA, a path timing can be detected by using the pilot symbol to detect a peak of a cross-correlation between a predetermined code sequence of the pilot symbol and a received signal. As a conventional path timing detection method for the W-CDMA, a xe2x80x9cReception Timing Detection Circuit for a CDMA Receiverxe2x80x9d disclosed in Japanese Patent Laid-Open No. 32523/1998, for example, is known.
In the W-CDMA, however, since the chip rate is higher than that of a system based on the IS-95 standards, a variation of the multi-path paths frequently varies the path timing discontinuously for more than one chip interval (because one chip is 60 ns, it corresponds to a propagation path difference of approximately 78 m). Accordingly, sufficient path tracking cannot be achieved by means of a DLL or a like scheme that is conventionally used for synchronization tracking (tracking) in a system based on the IS-95 standards or a like system.
Meanwhile, a path search circuit which uses the conventional path timing detection method has a problem in that, where the spread ratio of a pilot symbol is high and the code length of the pilot symbol is great, if a cross-correlation coefficient is calculated in the time domain, then a very great amount of calculation is required. Furthermore, a base station must detects path timings of received signals from a plurality of mobile stations, and the prior art has another problem in that a number of identical circuits equal to the number of mobile stations must be prepared and a great amount of calculation is required.
It is an object of the present invention to provide a path search circuit which can reduce the amount of arithmetic operation required for path search and path tracking in a mobile communications system (cellular system) that uses a DS-CDMA communication method.
It is another object of the present invention to provide a path search circuit which, when applied to a base station apparatus of a cellular system which uses a DS-CDMA communication method, can reduce the amount of arithmetic operation required for path search and path tracking of received signals from a plurality of mobile stations.
According to the present invention, a path search circuit for a receiver which uses a DS-CDMA communication method is for detecting a path timing, which is a timing at which spreading is performed on the transmission side, from a received radio signal and comprises a radio receiving unit, an A/D converter, a cross-correlation coefficient calculating unit, a cross-correlation coefficient averaging unit, and a peak detecting unit.
The radio receiving unit filters and frequency converts the received radio signal to convert the received radio signal into a baseband signal. The A/D converter samples the baseband signal at a sampling rate equal to N times a chip rate to convert the baseband signal into a digital signal.
The cross-correlation coefficient calculating unit includes interleave means, N fast Fourier transform means, reference signal storage means, N cross power spectrum calculating means, N inverse fast Fourier transform means, and deinterleave means.
The interleave means rearranges the baseband signal digitized by the A/D converter into N sequences sampled at chip intervals. The N fast Fourier transform means pick out the N received signal sequences rearranged by the interleave means with mutually overlapped FFT windows of a predetermined time length and performs fast Fourier transform for the picked out received signal sequences. The reference signal storage means stores a signal sequence produced by picking out a predetermined code sequence with FFT windows of a fixed time length and fast Fourier transforming the picked out code sequence as a reference signal.
The N cross power spectrum calculation means determine the product of the received signal fast Fourier transformed by the fast Fourier transform means and a complex conjugate number of the reference signal stored in the reference signal storage means for each of the FFT windows to determine cross power spectra between the received signal and the predetermined code sequence.
The N cross power spectrum averaging means averages the cross power spectra for the respective FFT windows. The N inverse fast Fourier transform means inverse fast Fourier transform the N cross power spectra averaged by the cross power spectrum averaging means to convert the cross power spectra into N cross-correlation coefficients and output the N cross-correlation coefficients.
The deinterleave means rearranges the N cross-correlation coefficients output from the respective inverse fast Fourier transform means in order of time to produce and output a single cross-correlation coefficient.
The cross-correlation coefficient averaging unit averages the cross-correlation coefficient output from the cross-correlation calculating unit over a fixed period of time. The peak detecting unit detects one or a plurality of peaks from the cross-correlation coefficient averaged by the cross-correlation coefficient averaging unit and outputs a timing at which the peak or each of the peaks is obtained as a path timing.
According to an embodiment of the present invention, in the path search circuit for a receiver which uses a DS-CDMA communication method, the value N may be an integer lowest among values which satisfy the relationship: Nxe2x89xa7radio bandwidth/chip rate.
According to another embodiment of the present invention, the path search circuit may further comprise interpolation means for oversampling the N cross-correlation functions output from the deinterleave means as sequences of a time interval equal to 1/N the chip interval to M times, where M is a positive integer, and passing the oversampled cross-correlation coefficients through a low-pass filter to produce cross-correlation coefficients oversampled to Nxc3x97M times the chip rate and then outputting the produced cross-correlation coefficients.
According to another aspect of the present invention, a path search circuit for a receiver which uses a DS-CDMA communication method comprises a radio receiving unit, an A/D converter, a cross-correlation coefficient calculating unit, a cross-correlation coefficient averaging unit, and a peak detecting unit.
The radio receiving unit filters and frequency converts the received radio signal to convert the received radio signal into a baseband signal. The A/D converter samples the baseband signal at a sampling rate equal to N times a chip rate to convert the baseband signal into a digital signal.
The cross-correlation coefficient calculating unit includes interleave means, N fast Fourier transform means, reference signal storage means, N cross power spectrum calculation means, N cross power spectrum averaging means, N first cross power spectrum conversion means, cross power spectrum addition means, second cross power spectrum conversion means, and inverse fast Fourier transform means.
The interleave means rearranges the baseband signal digitized by the A/D converter into N sequences sampled at chip intervals. The N fast Fourier transform means pick out the N received signal sequences rearranged by the interleave means with mutually overlapped FFT windows of a predetermined time length and perform fast Fourier transform for the picked out received signal sequences. The reference signal storage means stores a signal sequence produced by picking out a predetermined code sequence with FFT windows of a fixed time length and fast Fourier transforming the picked out code sequence as a reference signal.
The N cross power spectrum calculation means determine the product of the received signal fast Fourier transformed by the fast Fourier transform means and a complex conjugate number of the reference signal stored in the reference signal storage means for each of the FFT windows to determine cross power spectra between the received signal and the predetermined code sequence. The N cross power spectrum averaging means average the cross power spectra for the respective FFT windows. The N first cross power spectrum conversion means apply reflection by N times and phase rotation in the frequency domain to the N cross power spectra averaged by the cross power spectrum averaging means and having a bandwidth equal to the chip rate to convert the N cross power spectra into a single cross power spectrum having a bandwidth equal to N times the chip rate.
The cross power spectrum addition means adds the N cross power spectra converted by each of the first cross power spectrum conversion means. The second cross power spectrum conversion means adds the number of xe2x80x9c0xe2x80x9ds equal to Nxc3x97(Mxe2x88x921) times the chip rate to a high frequency of the cross power spectrum obtained by the addition means, where M is a positive integer. The inverse fast Fourier transform means inverse fast Fourier transforms the cross power spectrum obtained by the second power spectrum conversion means and having a bandwidth increased to M times to determine a cross-correlation coefficient.
The cross-correlation coefficient averaging unit averages the cross-correlation coefficients output from the cross-correlation calculating unit over a fixed period of time. The peak detecting unit detects one or a plurality of peaks from the cross-correlation coefficient averaged by the cross-correlation coefficient averaging unit and outputs a timing at which the peak or each of the peaks is obtained as a path timing.
According to the present invention, an advantage can be anticipated that arithmetic operation processing of cross-correlation coefficients essentially required for path search can be reduced by dividing a received signal into a plurality of FFT windows to perform fast Fourier transform and performing multiplication by a reference signal and averaging in the frequency domain. Particularly since the number of chips per FFT window can be increased by dividing a received signal into a plurality of sequences at one-chip intervals to perform fast Fourier transform, the overlap between FFT windows can be reduced. Since this decreases the number of FFT windows per received signal for a fixed time length, an advantage can be anticipated that the amount of arithmetic operation can be reduced.
If it is desired to raise the accuracy of a path timing higher than a sampling period of the A/D converter, A/D conversion is performed at the possible lowest sampling rate and cross-correlation coefficients or cross power spectra are determined, and then interpolation is performed in the time domain with a required time accuracy. This makes it possible to suppress the amount of arithmetic operation in fast Fourier transform operation, which involves a very great amount of arithmetic operation, to the possible lowest amount.
According to a further aspect of the present invention, a path search circuit for a receiver which uses a DS-CDMA communication method comprises a radio receiving unit, an A/D converter, a cross-correlation coefficient calculating unit, a cross-correlation coefficient averaging unit, and a peak detecting unit.
The radio receiving unit filters and frequency converts the received radio signal to convert the received radio signal into a baseband signal. The A/D converter samples the baseband signal at a sampling rate equal to N times a chip rate to convert the baseband signal into a digital signal.
The cross-correlation coefficient calculating unit includes interleave means, N fast Fourier transform means, reference signal storage means, cross power spectrum calculation means, cross power spectrum averaging means, inverse fast Fourier transform means, and deinterleave means.
The interleave means rearranges the baseband signal digitized by the A/D converter into N sequences sampled at chip intervals. The N fast Fourier transform means pick out the N received signal sequences rearranged by the interleave means with mutually overlapped FFT windows of a predetermined time length and perform fast Fourier transform for the picked out received signal sequences. The reference signal storage means is provided for each channel and stores a signal sequence produced by picking out a predetermined code sequence with FFT windows of a fixed time length and fast Fourier transforming the picked out code sequence as a reference signal. The N cross power spectrum calculation means are provided for each channel and determine the product of the received signal fast Fourier transformed by the fast Fourier transform means and a complex conjugate number of the reference signal stored in the reference signal storage means for each of the FFT windows to determine cross power spectra between the received signal and the predetermined code sequence. The N cross power spectrum averaging means are provided for each channel and average the cross power spectra for the respective FFT windows.
The N inverse fast Fourier transform means are provided for each channel, and inverse fast Fourier transform the N cross power spectra averaged by the cross power spectrum averaging means to convert the cross power spectra into N cross-correlation coefficients and output the N cross-correlation coefficients.
The deinterleave means is provided for each channel, and rearranges the N cross-correlation coefficients output from the respective inverse fast Fourier transform means in order of time to produce and output a single cross-correlation coefficient.
The cross-correlation coefficient averaging unit averages the cross-correlation coefficients output from the cross-correlation calculating unit over a fixed period of time. The peak detecting unit detects one or a plurality of peaks for each channel from the cross-correlation coefficient averaged by the cross-correlation coefficient averaging unit and outputs a timing at which the peak or each of the peaks is obtained as a path timing.
The present aspect of the invention corresponds to a path search circuit which is applied to a base station apparatus which must detect path timings of a plurality of channels (received signals from a plurality of mobile stations) simultaneously. According to the present aspect of the invention, it is required to perform fast Fourier transform operation for a received signal which requires the greatest amount of arithmetic operation only once irrespective of the number of reception channels.